Clinical use of quantitative X-ray methods such as the determination of bone mineral content or tissue density for dose planning purposes demand a knowledge of the energy distribution of the x-rays. Spectrometry with germanium detectors requires counting rats of about 50 000 pulses/second or less in order to avoid overlapping pulses (pile ups). With the high fluence from clinical x-ray tubes appropriate counting rates is attained only if the distance between the x-ray source and the detector are several meters and the collimators have diameters that are as small as 0.025-0.5 mm.
This direct measuring method cannot be used in practice in clinical x-ray laboratories due to space limitations. FIG. 2, the upper continuous line, shows a spectrum measured directly.
Due to the practical difficulties of the direct measuring method it is only used to a small extent. Moreover, there will be problems with the alignment of the detector because it is easy to measure, by mistake, beside or on the edge of the x-ray focus which, of course, is not intended.
An extremely efficient method for reduction of the pulse rate is utilizing Compton scattering, i.e., the radiation is scattered of the electrons in a material. However, when the radiation is scattered in this way energy is lost to the electrons and as a result of this the original spectrum will be distorted, the consequence being, in turn, that this is no longer representative. Accordingly, Compton scattering has not been utilized so far as a basis of spectrometry. FIG. 2, the lower curve, shows a Compton scattered spectrum.